There's an easy, flat walk in Tipner, beside the harbour on the north west part of Portsmouth that provides views back to the port, across to Portchester Castle, and as here, the land that was formerly a Royal Naval rifle range. It's currently quite a nice place to spot birds and butterflies in the summer but unused land doesn't stay unused for long around these parts.
The walk along the Langston Harbour shore that runs parallel to the Eastern Road is decent enough.
I really like what the city council have done with the sea defences. 😎